Bistable or multi-stable particle-based electrophoretic displays, and other electro-optic displays, have an optical behavior that is distinct from the optical behavior of conventional liquid crystal displays (“LCDs”). Twisted nematic liquid crystals, for example, act as instantaneous voltage transducers, so that applying a given electric field to the liquid crystal display medium produces a specific gray level in response to the applied voltage, regardless of the gray level previously present at the pixel.
Further, LCDs are only driven in one direction (from non-transmissive or “dark” to transmissive or “light”). The reverse transition, i.e., from a lighter state to a darker, is effected by reducing or eliminating the electric field. Hence, common LCDs are continuously driven in order to maintain a desired image.
Common LCDs also have pixel gray levels that are not sensitive to the polarity of a driving electric field. Indeed, commercial LCDs usually reverse the polarity of the driving field at frequent intervals. In contrast, bistable electro-optic displays act as impulse transducers, so that the final state of a pixel depends not only upon the magnitude and time of application of an electric field, but also upon the state of the display medium prior to the application of the electric field.
A method of controlling and applying well defined voltage impulses to an electro-optic medium is required to produce desired optical states in the medium. There are several ways of providing a particular voltage impulse, e.g., a particular ∫Vdt value, to a display medium. Two common methods entail modulation of the length of a constant voltage pulse, and modulation of the amplitude of a constant length pulse.
Amplitude modulation methods are commonly employed because such methods can provide, for example, reduced power consumption and reduced controller complexity. When an insufficient range of impulse control is possible using only amplitude modulation, amplitude modulation can be combined with time modulation to produce a more precise modulation of the total impulse applied to a display medium.
To control amplitude modulation at the pixel level in an active matrix display, a column driver circuit is typically required to adjust the amplitude of the driver circuit's output based on display signal data received from a display controller. A row driver circuit sequentially selects each row of pixels, temporarily connecting a selected row of pixel electrodes to the column driver circuits. In this way, the voltage of applied to each pixel electrode in the display can be set once per scan by the column and row drivers.
A column driver circuit commonly includes a resistive digital-to-analog converter (R-DAC) system with output buffers and offset trimming. Although a DAC-based architecture has many benefits, it typically requires a large number of transistors for implementation. This can lead to two problems: 1) the implementation of the circuit can be complex with care required to insure proper functionality and accuracy; and 2) a large area of active circuit can be required, which can lead to higher cost (especially at higher voltages.)
For example, a LCD having 256 gray levels may include a separate 256-level DAC for each column of display elements. Each DAC converts digital image data supplied to a column driver into a voltage to be applied to a pixel electrode. The cost of a large number of DACs in a high-resolution display may increase the manufacturing cost of a display.
Further, fabrication of an R-DAC-based design may require specialized process provisions, such as a floating polycrystalline silicon capacitor layer, to enable design features which improve accuracy. Specialized processes may reduce the number of vendors available with a suitable manufacturing capability and may increase final cost as well as the complexity and cost of designing the architecture.